Sometimes when we play a digital recording on a stereo the output sounds noisy. There are numerous factors that may make the sound emanating from our stereos more or less noisy.
Quantization of an audio signal to produce a digital recording introduces noise into the audio signal. The smaller the quantization step size, the less noise introduced. However, the smaller the quantization step size, the greater the number of bits required to store the digital recording.
Known techniques select quantization step sizes in such a manner so as to make the noise introduced by the quantization step size just imperceiveable to the human auditory system. Such a fundamental idea is discussed in U.S. Pat. No. 5,341,457 ("the '457 patent"), assigned to the assignee of the present invention and incorporated by reference herein as if set forth in its entirety. A technique described in the '457 patent determines a noise masking threshold (e.g., quantization step size) that, if used to encode, will introduce quantization noise that is just imperceiveable.
However, when the above technique is used for a multichannel, e.g., stereo signal, there are shortcomings. For example, in a stereo environment, the noise masking thresholds do not account for the effect on a listener of what is known as "noise unmasking." In stereo environment, "noise unmasking" occurs when noise introduced during quantization appears to the listener to emanate from a different direction than the sound the listener desires to hear. When this occurs, the quantization noise is said to be unmasked.
In attempting to solve the problem of noise unmasking, another technique was developed. Essentially, this technique describes adjusting noise masking thresholds of a sum channel signal ("S") and a difference channel signal ("D"). The adjusted noise masking thresholds provide additional protection against noise unmasking and, when selectively used, prove advantageous. However, the technique only adjusts noise masking thresholds for S and D (not for a left channel signal ("L") and a right channel signal ("R"). Thus, for encoders that do not use S and D to encode (i.e., encoders that encode always based upon L and R), this technique does not help. Examples of such encoders are encoders made in accordance with the standards referred to as MPEG Layers I and II. Further, even when used with S and D encoders, the technique may result in overcoding. This is because the technique does not always identify signals for which additional noise unmasking protection is unnecessary. Instead, it merely adds additional noise masking protection, based upon adjusted S and D masking thresholds, if the L masking threshold and the R masking threshold are similar.